In many processes for the chlorination or oxychlorination of ethylene, the conversion of ethylene is substantially less than complete. Thus, the effluent from a conventional oxychlorination reactor will contain, in addition to reaction products, from 0.1 percent to 15 percent or even 20 percent by weight unreacted ethylene, as well as HC1, O.sub.2, inert gases, etc. In view of the current environmental concern for maintaining hydrocarbon levels in the atmosphere as low as possible, as well as the high cost of ethylene, the recovery of the unreacted ethylene is a practical necessity.
Previous work in this area has evolved ethylene recovery systems which provided recovery of the majority of residual ethylene. French Pat. No. 1,421,903 and Belgian Pat. No. 718,777 disclose exemplary prior art procedures. However, as indicated, environmental and cost requirements now demand that virtually one hundred percent conversion of the feed ethylene be achieved.
Many current oxychlorination processes have attempted to solve this problem by the use of an ethylene clean-up reactor situated at the exit of the oxychlorination system. In one example of such a reactor, the ethylene in an oxychlorination effluent is reacted with chlorine to produce 1,2-dichloroethane (hereinafter referred to as ethylene dichloride or EDC), in the presence of an activated alumina catalyst. The EDC produced in the clean-up reactor can then be combined with that produced in the oxychlorination system. As the clean-up reactor, commercial plants commonly use a multi-tube reactor with a fixed bed catalyst. Inlet temperatures range from about 50.degree. C. to about 200.degree. C., and the temperature of the gas in the catalyst bed ranges from about 100.degree. C. to about 300.degree. C. The pressure ranges from about 15 psig to about 75 psig, the space velocity (which is defined as volume of gas at 0.degree. C. and atmospheric pressure per hour per volume of catalyst bed) ranges from about 500 to about 5000 hour.sup..sup.-1, and the contact time ranges from about 0.7 to about 32 sec. The chlorine is fed at about 5% molar deficit to about 10% molar excess with respect to the ethylene.
Small amounts of both chlorine and ethylene pass through the reactor unreacted and are present in the effluent. The amounts of each vary considerably with the chlorine-to-ethylene feed ratio. It is possible to maintain low levels of each by maintaining a constant feed ratio with a slight chlorine excess controlled to within one tenth of a percent. Such control would require continuous and accurate monitoring of the ethylene content in the ethylene-containing stream and highly accurate control of the chlorine feed rate. Such accuracy is difficult to achieve in commercial plant scale equipment, especially when fluctuations in operating conditions are encountered. Furthermore, the kinetics of the reactions place a lower limit on the amounts of each component which remain unreacted.
Emissions containing ethylene on the order of several thousand ppm and chlorine on the order of several hundred ppm are common in existing plants which use the ethylene clean-up system described above. The large excess of ethylene in these emissions may arise from a desire to avoid the consequences of chlorine in the atmosphere, examples of which are unpleasant odor and toxicity to plant life. A system was needed, therefore, which would convert a larger portion of the ethylene to EDC while avoiding the odor problem caused by chlorine. This goal is accomplished by placing a chlorine-removal system immediately downstream of the ethylene clean-up system described hereinabove and operating the combination at a greater chlorine-to-ethylene feed ratio. Chlorine removal is effectively achieved by catalyzing a reaction between chlorine and the hydrocarbons in the stream by addition or substitution reactions. In this manner, small amounts of hydrocarbons are chlorinated and partially chlorinated hydrocarbons are further chlorinated.
A variety of catalysts are known in the art to be active for this purpose, for example, alkaline earth chlorides, cupric chloride, and ferric chloride. Ferric chloride is known to catalyze oxychlorination or direct chlorination mechanisms, in both gas and liquid phase systems. Inert catalyst supports are frequently used in gas phase reactions to provide porosity and high surface area on which the reaction can take place. A recurring problem with a supported ferric chloride catalyst is the decline in activity over extended use of the catalyst. Although a precise reason for the decline is unknown, changes in the valence of the iron, the formation of iron oxides and volatilization of ferric chloride are possible contributors.
The objects of the present invention are to provide a chlorine removal system in which a high level of activity is maintained over an extended period of time, and to eliminate ethylene from an ethylene-containing stream by contacting the stream with chlorine in such a manner that both ethylene and chlorine are substantially eliminated from the vent gas.